In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine clothing or felt. Upon deposition, the white water forming a part of the stock, flows through the interstices of the felt, leaving a mixture of water and fiber thereon. The felt then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the nip press section of the papermaking process. In the nip press section, two or more cooperating rolls press the fibrous web as it travels on the felt between the rolls. The rolls, in exerting a great force on the felt, cause the web traveling thereon to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several vacuum and dewatering stages.
The amount of pressure applied to the web during the nip press stage is important in achieving uniform sheet characteristics. Variations in nip pressure can affect sheet moisture content and sheet properties. Excessive pressure can cause crushing of fibers as well as holes in the resulting paper product. Conventional methods addressing this problem have been inadequate, and thus, this problem persists in the nip press stage, often resulting in paper of poor quality, having uneven surface characteristics.
Roll deflection, commonly due to sag or nip loading, is a source of uneven pressure distribution. Rolls have been developed which monitor and alter the roll crown to compensate for such deflection. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
One such roll is described in U.S. Pat. No. 4,509,237. This roll has position sensors to determine the existence of an uneven disposition of the roll shell. The signals from the sensors activate support or pressure elements underneath the roll shell, thereby equalizing any uneven positioning that may exist due to pressure variations. The pressure elements comprise conventional hydrostatic support bearings which are supplied by a pressurized oil infeed line.
A similar roll is disclosed in U.S. Pat. No. 4,729,153. This controlled deflection roll has sensors for regulating roll surface temperature in a narrow band across the roll face. Other controlled deflection rolls such as the one described in U.S. Pat. No. 4,233,010 rely on the thermal expansion properties of the roll material, to achieve proper roll flexure. Such deflection compensated rolls are often useful for varying the crown. Thus, such rolls can operate as effectively at a loading of 100 pounds per inch as at 500 pounds per inch, whereas rolls without such capabilities can only operate correctly at a single specific loading.
Notwithstanding the problem of roll deflection, the problem of uneven loading across the roll length, and in the cross machine direction, persists since pressure is often unevenly applied along the roll. For example, if roll loading in a roll is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Methods have been used to uncover discrepancies in applied pressure. One such method requires stopping the roll and placing a long piece of carbon paper, foil, or impressionable film in the nip, which is known as taking a nip impression. However, one must load the rolls carefully to ensure that both sides, that being front and back, are loaded evenly. The pressure in the nip transfers a carbon impression, deforms the foil, or ruptures ink containing capsules in the film, indicating the width of contact. These methods for taking a nip impression are not reusable as they determine only a single event such as the highest pressure or contact width.
One of the major difficulties in using the nip impression procedure, is that of evenly loading the rolls from front to back. The goal of the procedure is to measure and record the final stable loading along the length of the rolls after the initial loading. Often during the initial loading, however, one end will contact before the other end. Thus, there are times when one end is heavily loaded while the other end is only slightly loaded. When this occurs, the nip impression shows the highly loaded condition and not the final state, since the carbon paper, foils, and prescale films record the largest width and/or highest pressures.
Another method of determining the nip pressure profile is to use a prescale film that measures pressure. The film is fed into the nip after the rolls are loaded. Therefore, the film records the stable loaded condition, rather than the worst consequence of the loading process. Such a process eliminates the loading difficulties associated with nip impressions. Nonetheless, the prescale films must be interpreted using a densitometer. This process is cumbersome, time consuming, and generally inefficient. Furthermore, the prescale films are not reusable. A new piece of film must be fed into the nip after any corrective adjustments are made. Lastly, the prescale films are temperature and moisture dependant, thus leading to inaccurate and unreliable results.